1. Field of the Invention
This invention is directed to an interlock for preventing the operation of a plasma-arc cutting system when necessary parts are not in place. It relates specifically to such an interlock system which senses flow rate in lines supplying working fluid such as gas to a plasma-arc torch which shuts off power to the torch when a necessary part is missing, as indicated by an increase in flow rate to a level above a predetermined amount.
2. Description of the Prior Art
Plasma-arc torches find wide application to tasks such as cutting, welding and spray bonding. These torches operate by directing a plasma consisting of ionized gas particles toward a workpiece.
In the operation of a typical plasma torch, such as illustrated in U.S. Pat. Nos. 4,324,971; 4,170,727; and 3,813,510 assigned to the same assignee as the present invention, a gas to be ionized is supplied to the front end of the torch in front of a negatively-charged electrode. The torch tip which is adjacent to the end of the electrode, at the front end of the torch, has a sufficiently high voltage applied thereto to cause a spark to jump between the electrode and the torch tip thereby heating the gas and causing it to ionize. A pilot DC voltage between the electrode and the torch tip maintains an arc known as the pilot, or non-transferred arc. The ionized gas in the gap appears as a flame and extends externally off the tip where it can be seen by the operator. As the torch head or front end is brought down towards the workpiece, the arc jumps from the electrode to the workpiece since that impedance of the workpiece current path is lower then the impedance of the torch tip current path.
The ionized gas or working fluid is supplied through a conduit from a source of fluid pressure to the torch tip. Frequently, a secondary flow of fluid is provided which passes through a separate flow path from the first mentioned working fluid for purposes of cooling various torch parts. In this case, the first mentioned fluid is called the primary fluid or gas and the second is called the secondary fluid.
Because the electrode and tip operate in a very high temperature environment, they must be replaced from time to time as they are used up. Accordingly, torches are designed to facilitate periodic replacement of these electrodes and tips.
Sometimes, because of operator carelessness perhaps, a tip, electrode or other essential torch part is left off the torch during replacement and not present when the torch is operated. This may cause operator injury. At the very least it can cause damage to the torch. For example, if the tip is not in place the arc generated from the electrode may strike and damage another part of the torch.
The assignee's own U.S. Pat. No. 4,585,921 issued Apr. 29, 1986, entitled "Torch Operation Interlock Device" describes an electrical circuit means that functions as an operation interlock when torch parts are not in place. If a sensed part is not in place, the control circuit functions to interrupt operation of the torch, thereby minimizing operator injury and torch damage.
While a satisfactory solution to the torch parts in place problem, applicant's assignee's prior art device does require a more complex electrical circuit. A current path must be established through the part or parts to be retained. This requires at least one additional wire to form a circuit. Such a circuit thus adds to cost as well as to complexity.